Parents of grown step children, how do you handle it when the kids are disrespectful to your spouse?

My husband’s kids are fairly regularly disrespectful to him. They take advantage of him and when they are having a rough time with things in their lives they tend to take it out on him. They consistently ignore his birthday and Father’s Day. They all got together today and no one bothered to invite him, much less wish him a Merry Christmas. He never mistreated these kids. He fought for six years to get custody and kept them out of foster care when they were taken from their alcoholic, abusive mother. The way they treat him infuriates me and makes me want to tell them off, but I know this is probably not a great idea. Does anyone have experience in this area? How have you handled it? Any advice would really be appreciated.